COSEWIC assessment and status report on the coho salmon (Interior Fraser population) in Canada
- Assessment Summary
- Executive Summary
- COSEWIC Mandate, Membership and Definitions
- Lists of Figures and Tables
- Species Information
- Population Sizes and Trends
- Limiting Factors and Threats
- Special Significance
- Existing Protection
- Summary of Status Report
- Technical Summary
- Acknowledgements, Literature Cited, and The Author
Interior Fraser Population
Coho salmon is one of seven species of the genus Oncorhynchus native to North America. Adult coho usually weigh from 2 - 5 kg (45 - 70 cm in length) and only rarely exceed 9 kg. Most coho spend their first year in freshwater and the next 18 months in the ocean before returning to freshwater to spawn and die. Jacks (precocious males) that spend only six months in the ocean are found in some populations.
The status of coho salmon from the interior Fraser River watershed (including the Thompson River) is evaluated in this document. The Fraser is the largest river in British Columbia (BC) and the interior Fraser (i.e. upstream of the Fraser canyon) constitutes most of the drainage basin. Interior Fraser coho occupy a significant proportion (~25%) of the range of coho salmon within Canada. Interior Fraser coho are genetically unique and can be distinguished from coho from the lower Fraser River watershed and other areas of Canada.
Coho salmon occur naturally only within the Pacific Ocean and its tributary drainage. The interior Fraser watershed is part of the Southern Mountain COSEWIC Ecological Area. Coho salmon are widespread throughout the Thompson River system, the largest watershed in the Fraser River system. Their distribution in non-Thompson tributaries of the interior Fraser is not well known. Coho are probably spawning in fewer streams within the interior Fraser than previously when they were more abundant. Coho salmon that were spawned in the interior Fraser River watershed have been recovered in fisheries from Alaska to Oregon, but most were caught off the West Coast of Vancouver Island and in the Strait of Georgia.
The distribution of spawning habitat for coho salmon is usually clumped within watersheds. Juvenile coho salmon tend to cluster in areas of suitable habitat in shallow gradient streams and sometimes lakes. Much of the interior Fraser watershed where coho are found has been logged and is used for a variety of agricultural activities.
Juvenile coho salmon migrate down the Fraser River and spend an unknown time in the highly developed Fraser River estuary. The majority of their oceanic residence is usually spent near the coast in southern BC. Although marine areas used by Fraser coho are relatively undeveloped by humans, climate-related changes have reduced the ability of the marine environment to support these fish in southern BC in many recent years.
Interior Fraser coho salmon return to freshwater in the fall and spawn during fall and early winter. Fry emerge from the gravel the following spring and usually reside in freshwater for a year before migrating to sea as smolts. Almost all coho spend 18 months at sea before returning to freshwater and therefore have a 3-year life cycle.
Female coho salmon are larger than males in most interior Fraser systems, but less abundant (~45% of returns). Interior Fraser coho are smaller and usually less fecund than most similar-aged coho. Temporal patterns in size have not been documented.
Population Sizes and Trends
Our time series of reliable estimates of spawners begins in 1975. Spawner numbers in the North and South Thompson watersheds peaked in the mid-1980’s, declined rapidly until about 1996, and have been stable or potentially increasing since then. Slightly more than half of recent estimates of the total population of 24 000 occur within the North and South Thompson watersheds. Most coho salmon returning to the interior Fraser are produced by natural spawning (~20 000 of ~24 000 total, mean of 1998-2000 estimates). Decline estimates for the 1990-2000 10-year period averaged 60%. Peak escapements and abundances during the mid-1980’s (100 000 and 300 000 respectively) were somewhat less than crude estimates derived for the 1920’s and 1930’s (200 000 and 400 000).
Recent marine survivals have been 3% or less, much lower than during the 1970’s and 1980’s. Fishery exploitations (proportion of adults caught in fisheries) averaged 68% until 1996. In response to conservation concerns, exploitations were reduced to ~40% in 1997 and averaged 6.5% the next three years.
Productivity declined between the 1980’s and the 1990’s. There were four years (1991, 1995, 1997, and 1998) when some populations may not have been able to replace themselves, even in the absence of fishing. Spawner numbers in 1999 and 2000 have exceeded parental escapements. However, the outlook for interior Fraser coho is highly uncertain and depends on fishing, habitat perturbations, and climate-related changes in survival.
Limiting Factors and Threats
Overfishing, changing marine conditions, and habitat perturbations all contributed to declines. Excessive fishing resulted when harvest rates were not reduced quickly in response to climate-driven declines in marine productivity. In addition, coho declines were often related to the intensity of human disturbance in freshwater.
Special Significance and Existing Protection
Coho salmon remain an important species, contributing to catches along the Pacific coast of North America. Numbers of coho are declining throughout much of its range. In the United States, coho salmon are considered to be threatened by extinction in three Evolutionarily Significant Units (ESUs), candidates for listing in two ESU, and not likely to become endangered in only one ESUs. Stock status for coho in BC varies depending on location, but coho from the interior Fraser appear to have declined at a greater rate than coho from other areas.
The responsibility for managing salmon and salmon habitat in BC is shared between the federal and provincial governments. A variety of legislative processes, some international, are in place to ensure salmon conservation. The federal Fisheries Act is a powerful piece of legislation providing the authority for the management and regulation of fish and fish habitat. Recent regulatory changes made to conserve interior Fraser coho salmon were probably the most significant fishery changes ever implemented within the Pacific Region of Canada. Since there is no consensus regarding future marine survivals for interior Fraser coho salmon, a continuing and extremely cautious approach to managing both fisheries and habitat will be required to ensure the long-term viability of these fish.
Summary of Status Report
This report focuses on coho salmon from the interior Fraser River of British Columbia. These genetically distinct salmon constitute an Evolutionarily Significant Unit made up of at least five subpopulations. Slightly more than half of recent estimates of the total population of 24 000 (~20 000 wild) occur within the North and South Thompson watersheds. The best abundance indicators are spawner estimates for the North and South Thompson watersheds, which peaked in the mid-1980’s, declined until about 1996, and have been stable or increasing since then. Rates of decline for 1990-2000 averaged 60%. There is no evidence that the extent of occurrence has changed, although spawners were seen in fewer streams as populations declined. The main reason for the decline in numbers of interior Fraser coho salmon is excessive fishing that resulted when harvest rates were not reduced quickly in response to climate-driven declines in marine survival. Freshwater habitat degradation also played a role -- coho declines were related to the intensity of human disturbance in the watershed. Fishing pressures have been reduced dramatically the last several years, and this combined with an apparent stabilization in marine survivals resulted in improved returns. But the outlook for interior Fraser coho is highly uncertain and will depend on impacts due to fishing, habitat perturbations, and climate-related changes in survival. An extremely cautious approach to managing both fisheries and habitat is required to ensure the viability of populations of coho salmon within the interior Fraser River watershed.
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